Calculating BTU’s Per Square Foot

The British Thermal Unit, or BTU, is the Imperial system’s basic unit of heat. One BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. When you’re able to determine how many BTUs per square foot of space a room requires, you’ll can pick a heating device with the right heating capacity for your home.

Close up of calculator

Photography by Jan Kaláb

Determining How Much to Heat

The first step in calculating the BTUs you need is to calculate how much space you have in a room. Some common room shapes in a home are rectangular, triangular, circular, and multiple shapes.

  • Rectangular Rooms – Measure both the length and width of the room in feet, and multiply both numbers to get your square footage.
  • Triangular Rooms – Measure both the length and width of the room in feet, and multiply both numbers and divide the product by 2 to get your square footage.
  • Circular Rooms – Measure the radius of the room (the distance from the center to the edge). Multiply the radius with itself, and then by ? to get your square footage. If a room contains a semicircle piece, find the square footage as if it were a regular circle and then divide by two.
  • Rooms with Multiple Shapes – Split the room up into multiple shapes, find the square footage of each individual shape, and then sum the square footages.

BTUs of heat or cooling devices like electric fireplaces or air conditioners are communicated as “BTUs,” but this is shorthand for BTUs per hour. To determine the number of BTUs per square foot that you need to heat a room, simply multiply the square footage by 20 BTUs per square foot. For example, if a room has 1,000 square feet, you would require 20,000 BTUs to heat it.

While this method is simple, it does not take into account the insulation, age, or climate of your home.

Other Factors that Effect Heating Capacity

A room’s heating capacity is the number of BTUs required to heat it properly. Calculating the room’s heat capacity includes applying the effects of the home’s age, insulation, and climate that it’s in.

  • Age and Insulation – Due to revised building codes, newer buildings typically are better insulated than older ones. As a result of this, they require fewer BTUs per hour per square foot.
  • Climate – Warmer climates require fewer BTUs to properly heat the home (30-35 BTUs), while colder climates require more BTUs to provide adequate heat (50-60 BTUs).
BTU Heating Capacity chart

© STL Stores Inc.

Example 1)

In the figure below, the room on the bottom-right is 12′ 4″ x 18′ 4″, or 12.3′ x 18.3′. If you calculate the square footage of the room, it totals to about 225 square feet.

Apartment Floor Plan

Floor plan courtesy Dhlinva. Click to enlarge.

Let’s say the room you’re trying to heat is in a new house in San Diego: the room’s heating factor would be about 30 BTU per square foot, since it should be an insulated room in a warmer climate. If you multiply the heating factor by the square footage, you’ll get about 6,750 BTU per hour.

Efficiency of Heating Devices

Heat devices are rated by the amount of BTUs you receive, not the amount of BTUs it generates. How much of the generated input heat that reaches you as output heat is also known as the efficiency of the heating device. This efficiency is shown as a percentage, typically as a ratio of output heat to input heat.

A device such as a modern electric fireplace is rated as 80 or 90 percent efficient. To apply this to your heat capacity, divide the heat capacity of the room by the device’s efficiency. In the example above, 6,750 BTU per hour divided by 0.9 is 7,500 BTU per hour. With this information, we can now see that to heat the room properly, you will need to have a fireplace rated to 7,500 BTU per hour input.

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Purchasing Heating Devices

A fireplace is more than an aesthetic addition to your home; it’s a functional piece of furniture that will keep you and the family warm. has a variety of electric fireplaces and outdoor heaters ready to keep you warm in the nights to come. Contact one of our heating experts today for assistance placing an order by calling 1-888-682-2591 or sending us a message.

More Tips on Calculating Home Heating

  1. Determining How Many BTU’s Your Home Needs
  2. Understanding BTU Readings – The Simplified Version
  3. Electric Fireplaces that Heat 1,000 sq ft Spaces
  4. Understanding Electric Readings-Watts, Amps, Volts, & Ohms
  5. Heating Efficiency Put to the Test: Wood, Electric, and Gas Fireplaces